Continuous shroud for rotor blades



Feb. 6, 1968 A. J. PARTINGTON CONTINUOUS SHROUD FOR ROTOR BLADES Filed Dec. 19, 1966 FIG. I

INVENTOR Albert J. Purfingion BY WITNESSES MP W 2 United States Patent Ofiice 3,367,629 CONTINUOUS SHROUD FOR ROTOR BLADES Albert J..Partington, Media, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 19, 1966, Ser. No. 602,652 8 Claims. (Cl. 253-77) ABSTRACT OF THE DISCLOSURE This invention provides a shrouded blade structure for an elastic fluid axial flow turbine or compressor and comprises two annular concentric layers of staggered segments disposed in mutual face-to-face abutment with each other and firmly connected to the blades by rivets formed on the blades. The blades are firmly connected in pairs by the segments. This arrangement provides frictional damping due to the area of contact between the layers of segments and a reduction in thermal stresses due to expansion of the shroud in operation.

This invention relates, generally, to elastic fluid axial flow apparatus and, more particularly, to shroud structures for blades utilized in the rotors of such apparatus.

Vibrational stresses in the blades of turbine rotors are dependent upon the excitation and the vibratory response of the blade structural system to the excitation. The vibratory response or magnification varies with the ratio of the excitation frequency to the blade natural frequency (nearness to resonance) and system damping. The resonant peak amplitude occurs when a frequency of the exciting force, or any multiple of it, is equal to the natural frequency of the blade.

Practical structural features cause variations in the motive fluid flow (for example, steam in steam turbines) which are the predominant source of excitation. The nature of these flow variations is a periodic wave with a fundamental frequency equal to the turbine operating speed, and also many harmonics. The magnitude of the excitation increases with increased steam flow.

Grouped turbine blades have several different natural vibration modes. The first or tangential mode is an inphase vibration in the plane of maximum flexibility perpendicular to the axis of the unit. The higher frequency second mode vibration is also in-phase with the deflection essentially in an axial direction. A vibration of the blade groups in approximately an axial direction with one end of the group out of phase with the other is the third or torsional mode.

An object of this invention is to preclude group modes of vibration of rotor blades by providing a continuous shroud for each row of blades.

Another object of the invention is to connect all the blades of an annular row of blades into a continuously connected segmented shroud.

A further object of the invention is to increase the area of contact between parts of a shroud structure, whereby any relative motion will produce additional frictional damping.

Other objects of the invention will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with one embodiment of the invention, the outer ends of all blades of an annular row of turbine blades are attached to a continuously connected segmented shroud which comprises two annular concentric layers of staggered arcuate segments alternately connecting the blades in pairs. Thus, each blade is connected to the adjacent blade on one side by a segment in the outer layer and to the adjacent blade on the other side by a segment in the inner layer of the two concentric layers.

3,367,629 Patented Feb. 6, 1968 The connecting segments of each layer are similar in shape.

For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a view, in elevation, of a portion of a turbine rotor having a shroud constructed in accordance with the present invention;

FIG. 2 is a view, partly in elevation and partly in section, taken along the line II-II in FIG. 1;

FIG. 3 is an enlarged view, in elevation, of the shroud structure shown in FIG. 1; and

FIG. 4 is an enlarged developed plan of the shroud structure.

Referring to the drawing, and particularly to FIG. 1, the structure shown therein includes a portion of a rotor core 10, a transverse annular row of blades 11 and a shroud 12 surrounding the outer ends or tips of the blades 11, which blades may be of the usual air foil contour. The blades 11 are attached to the periphery of the rotor core 10 in any suitable manner, for example by means of root portions 13 disposed. in a suitable peripheral groove in the core, as best shown in FIG. 2. The blades 11 extend radially outwardly with respect to the longitudinal axis of the rotor and they may constitute one stage of a multi-stage axial flow turbine.

In order to provide a continuous shroud which joins all of the blades 11 over 360, the shroud 12 comprises two annular concentric layers of staggered arcuate segments 14 and 15 alternately connecting the blades 11 in pairs. As shown more clearly in FIG. 2, each blade 11 has an integrally formed rivet 16 which extends from an integral cover 17 on the blade through the inner segment 15 and the outer segment 14. The connecting segments of each layer are similar in shape and the segments of the two layers are of substantially the same width. Each segment is of sufficient length to span two of the blades 11. The segments are disposed in staggered relation with the segments in the outer layer covering the juxtaposed end portions of the segments in the inner layer. Thus, each blade 11 is connected to the adjacent blade on one side by a segment 14 in the outer layer and to the adjacent blade on the other side by a segment 15 in the inner layer of the two concentric layers.

For ease of assembly and also in order to provide for thermal expansion of the shroud, spaces 18 are provided between the juxtaposed ends of the segments in each layer. Since the segments in the outer layer overlap the segments in the inner layer, each space 18 is either covered by a segment or it overlies a segment so that there are no gaps directly through the shroud 12. In this manner, better sealing against leakage of motive fluid by the shroud is obtained. The blade covers 17 are of parallelogram shape (in plan) and curved to conform to the curvature of the shroud segments 15 and are disposed in mutually abutting relation. In a similar manner, the bases 19 of the blades, which are similar in shape to the blade covers 17 are disposed in mutually abutting relation. Thus, the blades 11 are firmly held in position in the rotor 10.

One advantage of the continuous shroud is the substantial elimination of group modes of vibration. The only vibration which can occur are axial and tangential modes with an integral number of sine waves around the circumference. These have a similarity to disk vibration in that nodal diameters characterize the different modes.

Another advantage of the double layer shroud is the increased area of contact between parts of the shroud, whereby any motion will produce additional frictional damping. Although thermal stresses are not completely eliminated, any stresses developed during operation would be tolerable.

The shroud hereinbefore described may be utilized with turbine blades of a type previously manufactured. The arcuate segments of the shroud may be assembled on the rotor blades by peening over the rivets 16. Thus, the shroud may be economically manufactured and installed.

Since numerous changes may be made in the abovedescribed construction, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In elastic fluid utilizing apparatus, a rotor having a transverse annular row of blades, a shroud surrounding the outer ends of the blades, said shroud comprising two annular concentric layers of arcuate segments disposed in mutual abutment with each other, and connecting means on each blade extending through the layers of segments to alternately connect the blades in pairs, said connecting means firmly connecting said segments to said blades.

2. The structure defined in claim 1 wherein the connecting means is integral with each blade.

3. The structure defined in claim 1 wherein the segments of each layer are similar in shape.

4. The structure defined in claim 2 wherein the connecting means is a rivet extending longitudinally of the blade.

5. The structure defined in claim 1 wherein each blade is connected to the adjacent blade on one side by a segment in the Outer layer and to the adjacent blade on the other side by a segment in the inner layer of the two concentric layers.

6. The structure defined in claim 5 wherein the segments in the two layers are of substantially the same width.

7. The structure defined in claim 5 wherein spaces are provided between the ends of the segments in each layer.

8. The structure defined in claim 7 wherein the spaces between the segments in the inner layer are covered by the segments in the outer layer.

References Cited UNITED STATES PATENTS 1,129,920 3/1915 Thompson 253-77 1,842,957 1/1932 Bassler 25377 FOREIGN PATENTS 229,478 1925 Great Britain.

EVERETTE A. POWELL, JR., Primary Examiner. 

